A team of Indian astronomers has discovered that UBC 63, previously classified as a single star cluster, is actually made up of two distinct stellar systems that passed remarkably close to each other several million years ago. The discovery offers new insights into how gravitational interactions influence the structure and evolution of star clusters in the Milky Way.
Located approximately 4,892 light-years from Earth, UBC 63 is a loosely gravitationally bound group of stars known as a galactic open cluster. Earlier research had treated it as a single cluster, with only limited information available about its properties.
The latest study, however, reveals that the system consists of two separate clusters, designated UBC 63A and UBC 63B. Researchers identified the two populations using data from the European Space Agency’s (ESA) Gaia mission, which provides highly precise measurements of stellar positions and movements across the Milky Way.
The discovery provides an opportunity to investigate the aftermath of a close encounter between two stellar systems and understand how such interactions affect their long-term evolution.
Gaia Data Reveals Two Distinct Stellar Populations
The research began as an investigation into the star formation dynamics of UBC 63. A team led by Indian astronomers from Gauhati University and the Indian Institute of Astrophysics (IIA), an autonomous institution under the Department of Science and Technology (DST), used Gaia observations to examine the cluster’s stellar properties.
The researchers initially noticed that the stars appeared to form two separate sequences in a colour-magnitude diagram, an important astronomical tool used to study the brightness, colours and evolutionary stages of stars.
This observation suggested that UBC 63 might contain two distinct stellar populations rather than one unified cluster.
Further examination of stellar movements and other properties, supported by machine learning techniques and advanced dynamical simulations using high-performance computing, confirmed the presence of two separate clusters.
The analysis also revealed a substantial difference in their ages. UBC 63A is estimated to be approximately 21 million years old, while UBC 63B is considerably older, with an estimated age of around 562 million years.
This significant age difference indicates that the two clusters did not form as a single stellar system.
Two Star Clusters Passed Within 23 Light-Years of Each Other
After identifying the two clusters, researchers investigated how stellar systems with such different formation histories came to occupy nearby regions of the Milky Way.
By simulating their movements through the galaxy, the team reconstructed their past trajectories and identified evidence of a remarkably close encounter.
According to the simulations, when the younger cluster formed, the two systems were separated by approximately 60 parsecs, equivalent to around 196 light-years.
However, approximately six million years ago, they approached each other to within just seven parsecs, or roughly 23 light-years.
Today, UBC 63A and UBC 63B are separated by approximately 26 parsecs, equivalent to around 85 light-years.
Their present movements indicate that the clusters are not gravitationally bound to one another. Instead, they are moving apart following their close encounter.
Such interactions between star clusters are relatively uncommon, making the discovery particularly useful for studying how gravitational encounters influence stellar systems.
Study Highlights the Role of Gaia and Machine Learning in Astronomy
The discovery demonstrates how modern astronomical surveys can reveal structures that may remain hidden when stars are examined only through their apparent positions in the sky.
Gaia’s precise measurements of stellar positions and movements allowed researchers to distinguish the two populations within what was previously considered a single cluster.
The use of machine learning and high-performance computing further helped the team examine stellar properties and reconstruct the clusters’ dynamical histories.
The findings provide an opportunity to study how close encounters between stellar systems influence their structure, movement and evolution within the Milky Way.
Indian Research Team Publishes Findings in The Astronomical Journal
The research was published in The Astronomical Journal and involved an international team of astronomers.
The authors include Samrat Biswas, an INSPIRE Senior Research Fellow at Gauhati University; Biman J. Medhi of Gauhati University; Sergio Messina of the INAF Catania Astrophysical Observatory; Zhanpeng Zhu of Shanghai Normal University; Songmei Qin of the Shanghai Astronomical Observatory; Sukanta Deb of Cotton University; A. H. Sheikh, also an INSPIRE Senior Research Fellow at Gauhati University; H. S. Das of Assam University; Motohide Tamura of the University of Tokyo; Maheswar Gopinathan and Ram Sagar of the Indian Institute of Astrophysics.
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